Programmable conductor memory cell structure and method therefor

ABSTRACT

In programmable conductor memory cells, metal ions precipitate out of a glass electrolyte element in response to an applied electric field in one direction only, causing a conductive pathway to grow from cathode to anode. The amount of conductive pathway growth, and therefore the programming, depends, in part, on the availability of metal ions. It is important that the metal ions come only from the solid solution of the memory cell body. If additional metal ions are supplied from other sources, such as the sidewall edge at the anode interface, the amount of metal ions may not be directly related to the strength of the electric field, and the programming will not respond consistently from cell to cell. The embodiments described herein provide new and novel structures that block interface diffusion paths for metal ions, leaving diffusion from the bulk glass electrolyte as the only supply of metal ions for conductive pathway formation.

RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/121,790, entitled “PROGRAMMABLE CONDUCTOR MEMORY CELL STRUCTUREAND METHOD THEREFOR,” filed Apr. 10, 2002.

FIELD OF THE INVENTION

This invention relates generally to memory devices for integratedcircuits and more particularly to an anode contact for a programmableconductor random access memory (PCRAM) cell.

BACKGROUND OF THE INVENTION

The digital memory chip most commonly used in computers and computersystem components is the dynamic random access memory (DRAM), whereinvoltage stored in capacitors represents digital bits of information.Electric power must be supplied to the capacitors to maintain theinformation because, without frequent refresh cycles, the stored chargedissipates, and the information is lost. Memories that require constantpower are known as volatile memories.

Non-volatile memories do not need frequent refresh cycles to preservetheir stored information, so they consume less power than volatilememories. The information stays in the memory even when the power isturned off. There are many applications where non-volatile memories arepreferred or required, such as in lap-top and palm-top computers, cellphones or control systems of automobiles. Non-volatile memories includemagnetic random access memories (MRAMs), erasable programmable read onlymemories (EPROMs) and variations thereof.

Another type of non-volatile memory is the programmable conductor orprogrammable metallization memory cell, which is described by Kozicki etal. in (U.S. Pat. Nos. 5,761,115; 5,914,893; and 6,084,796) and isincorporated by reference herein. The programmable conductor cell ofKozicki et al. (also referred to by Kozicki et al. as a “metal dendritememory”) comprises a glass ion conductor, such as a chalcogenide-metalion glass, and a plurality of electrodes disposed at the surface of thefast ion conductor and spaced a distance apart from one another. Theglass/ion element shall be referred to herein as a “glass electrolyte”or, more generally, “cell body.” When a voltage is applied across theanode and cathode, a non-volatile conductive pathway (considered asidewall “dendrite” by Kozicki et al.) grows from the cathode through oralong the cell body towards the anode. The growth of the dendritedepends upon applied voltage and time; the higher the voltage, thefaster the growth rate; the longer the time, the longer the dendrite.The dendrite can retract, re-dissolving the metal ions into the cellbody, by reversing the polarity of the voltage at the electrodes.

In the case of a dielectric material, programmable capacitance betweenelectrodes are programmed by the extent of dendrite growth. In the caseof resistive material, programmable resistances are also programmed inaccordance with the extent of dendrite growth. The resistance orcapacitance of the cell thus changes with changing dendrite length. Bycompletely shorting the glass electrolyte, the metal dendrite can causea radical change in current flow through the cell, defining a differentmemory state.

For the proper functioning of a memory device incorporating such achalcogenide-metal ion glass element, it is important that growth of theconductive pathway have a reproducible relationship to applied voltage.For device operation, multiple cells across an array should ideally havea consistent response to the signals they receive.

The current invention addresses the issue of consistent memory cellresponse by ensuring a uniform supply of metal ions for formation of aconductive pathway under applied voltage.

SUMMARY OF THE INVENTION

A programmable conductor memory cell for an integrated circuit isdisclosed. In accordance with one aspect of the invention, the memorycell includes a memory cell body, formed from a glass electrolyteelement having metal ions disposed therein, which fills a cell body viain a first insulating layer. A cathode is in contact with the cell bodyat the bottom of the cell body via. The second insulating layer, whichoverlies the first insulating layer and the cell body, has an anode viatherein that is positioned concentrically over the memory cell body. Theanode via is filled with anode material so that the anode contacts onlya central portion of the anode surface of the memory cell body, whichcentral portion is spaced inwardly from the sidewall of the memory cellbody.

In a preferred embodiment, the anode via is lined with a spacer,preferably of insulating material, to ensure coverage of the sidewalledge of the memory cell body. In another embodiment, the anode via isformed using a mask with an opening smaller in width than the memorycell body and having the opening arranged concentrically over the memorycell body. In this way the sidewall edge of the memory cell body iscovered by the second insulating layer.

The memory cell body can comprise a chalcogenide glass electrolytematerial, preferably germanium-selenium, containing metal ions such assilver.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be better understood fromthe description below and the appended drawings, which are meant toillustrate and not to limit the invention, and in which:

FIG. 1 a is a cross section of a partially fabricated programmableconductor memory cell for an integrated circuit, constructed inaccordance with a preferred embodiment of the present invention.

FIG. 1 b is a perspective view of the partially fabricated programmableconductor memory cell of FIG. 1 a.

FIG. 1 c is cross section of a partially fabricated memory cell for andintegrated circuit, constructed in accordance with another embodiment ofthe present invention.

FIG. 2 is a cross section showing an embodiment of the current inventionwherein an anode via has a smaller diameter than the memory cell bodyand is formed concentrically thereover.

FIG. 3 is a cross section showing the programmable conductor memory cellof FIG. 1 a after deposition of an insulating layer, formation of ananode via therein and deposition of a conformal layer of siliconnitride, according to another embodiment of the current invention.

FIG. 4 is a cross section showing the programmable conductor memory cellof FIG. 3 after a spacer etch has been performed.

FIG. 5 is a cross section showing the structure of FIG. 3, after a metallayer has been deposited into the spacer-lined anode via.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

For proper functioning of a “programmable conductor” memory cell device,incorporating a glass electrolyte element with an adjustableconductivity, it is important that the conductive pathway growth inresponse to a particular applied voltage occurs reproducibly andconsistently across an array. Low voltages cause slow growth, whereashigher voltages result in faster growth of the conductive path. Theamount of growth in a given switching time depends, in part, on theavailability of metal ions. Therefore, it is important that the cationscome from a controlled source, such as from the solid solution of thecell body or glass electrolyte, which supplies an amount of cationsproportional to the concentration therein and to the electric field. Ifadditional cations are supplied from other, less reliable sources, theamount of cations may not be directly and reproducibly related to thestrength of the electric field or switching time.

For example, the interface between the cell body sidewall and thesurrounding insulating layer can provide a diffusion path for metalatoms and ions. When a metal anode layer (e.g., silver) is in contactwith the edge (shown in FIG. 1 b as 115) of the cell body sidewall,i.e., where the sidewall makes contact with the anode surface, there isadditional diffusion of metal cations along the sidewall, through theinterface, to the growing conductive pathway. If the anode via isdesigned to have the same width as the cell body via, even slightvariations in mask registration can result in large differences in thecontact area between the anode and the edge of the cell body sidewall,regardless of conventional mechanisms to minimize the effect of maskmisalignment. These differences in contact area lead to differences inthe metal supply through the cell body/insulator interface to thegrowing conductive pathway. Thus, the extent of conductive pathwayformation would depend not just on applied voltage and/or switchingtime, but also on the amount of metal leakage along the sidewall.Accordingly, the preferred embodiments provide means for avoidingdifferential contact area between the anode and the edge of the glasselectrolyte element.

A preferred embodiment of the current invention can be describedbeginning with reference to FIG. 1 a, wherein the first components of asimplified programmable conductor memory cell for an integrated circuitare shown. A cathode layer 101, which is connected to the negative poleof a power supply, is shown. Preferably, the cathode layer 101 comprisestungsten (W). An insulating layer 103, preferably silicon nitride(Si₃N₄), is deposited over the cathode layer 101. In other arrangements,it will be understood that the thick planarized insulating layer 103 cancomprise a form of silicon oxide, such as TEOS or BPSG, although it ispreferred to define the sidewall with a material that prevents thediffusion of metal between devices. The thickness of the insulatinglayer 103 is preferably between about 10 nm and 200 nm, more preferablybetween about 25 nm and 100 nm and most preferably about 50 nm. A cellbody via 105 is etched through the insulating layer 103, opening awindow to the cathode layer 101, using standard patterning and etchingtechniques. The width of the cell body via 105 is preferably betweenabout 100 nm and 500 nm, more preferably between about 200 nm and 300nm, and most preferably about 250 nm. The cell body via 105 is filledwith a glass electrolyte 107 (sometimes referred to in the literature asa Glass Fast Ion Diffusion or GFID element). The illustrated cell bodypreferably includes a chalcogenide glass, more preferably a glasscomprising germanium and selenium (Ge—Se) and most preferably, Ge₄Se₆,Ge₃Se₇ or Ge₂Se₈, and additionally includes metal ions. The actualratios of elements in the cell body 107 can vary and more complicatedstructures for the cell body 107 are also contemplated, one of which isillustrated in FIG. 1 c and discussed below. Once the cell body via 105is filled, the top surface 109 of the Ge—Se 107 is made level with thetop surface 111 of the insulating layer 103, preferably by chemicalmechanical planarization. Preferably the height of the programmableconductor memory cell body between the cathode surface and the anodesurface is in the range of about 25 nm to 100 nm.

Some aspects of the glass electrolyte element that are helpful forunderstanding the embodiments of the current invention are shown in FIG.1 b, a perspective view of the first components of the programmableconductor memory cell already seen in cross section in FIG. 1A. Theglass electrolyte element 107 is shown embedded in the insulating layer103 and making contact with an underlying cathode layer 101. Thesidewall 113 of the glass electrolyte element is defined as the outer,cylindrical (in the illustrated embodiment) surface of the element,which is defined by the surrounding via wall 105. The edge 115 of thesidewall 113 is the intersection of the glass electrolyte elementsidewall 113 and the top surface 109. In the illustrated embodiment, theedge 115 of the sidewall 113 has the form of a circle.

In the illustrated embodiment, in order to supply metal ions to theGe—Se glass, a thin layer (not shown) of metal or a combination ofmetals, including metal(s) from Group IB or Group IIB, more preferably,silver, copper or zinc, is preferably deposited over a recessed topsurface 109 of the fast ion conducting element and metal ions are driveninto the glass. The thickness of the metal layer is between about 2 nmand 10 nm, more preferably between about 3 nm and 8 nm and mostpreferably about 5 nm. For example, silver (Ag) ions can be driven intothe Ge—Se material by exposing an overlying Ag layer to ultravioletradiation with a wavelength less than 50 nm or through plasma treatment.Preferably, there is enough silver available in the layer to form aternary compound, silver germanium selenide, which is a stable amorphousmaterial. Silver constitutes preferably between about 20% and 50%, morepreferably between about 25% and 35% and most preferably about 30%(atomic percent) of the compound. The ternary compound is a glasselectrolyte material. The amount of silver formed over the glass ispreferably selected to be completely consumed by the photodissolutionprocess. After formation of the glass electrolyte material, the topsurface 111 can be planarized again to remove any remaining metal.

In other arrangements, metal for the programmable conductor memory issupplied by other means. For example, a layer containing a mixture oftungsten-silver of about 50%—50% by weight can be co-sputtered onto theglass electrolyte as a source of silver ions. In still otherarrangements, the metal and glass material can be co-sputtered ordeposited from a source that contains all species, so no metaldeposition and drive-in steps are needed.

FIG. 1 c illustrates another arrangement of the cell body 107, whereinlike reference numerals are employed to refer to like parts among thedifferent embodiments. In this arrangement, the cell body 107 includesthree layers, comprising a first Ge—Se layer 107 a (e.g., Ge₄Se₆), ametal selenide layer 107 b (e.g., Ag₂Se) and a second Ge—Se layer 107 c(e.g., Ge₄Se₆). The skilled artisan will appreciate that the embodimentsdiscussed below are equally applicable to forming electrodes over thecell body 107 of FIG. 1 a, FIG. 1 b or of any of a variety of otherprogrammable conductor arrangements. In the illustrated embodiment ofFIG. 1 c, the intermediate layer 107 b provides metal to the cell body107 for formation of conductive pathways under the influence of appliedelectrical fields. The structure can be formed by blanket deposition andetch or by first forming and then filling a via. In either case, thesidewall of the insulator surrounding the cell body is referred to as a“via” herein.

Regardless of how formed, the cell body or glass electrolyte element107, including metal ions diffused therein, serves as the memory cellbody.

With reference to FIG. 2, a second insulating layer 121, preferablysilicon nitride, is deposited over the first insulating layer 103. Thethickness of the second insulating layer 121 is preferably between about50 nm and 200 nm, more preferably between about 80 nm and 150 nm andmost preferably about 100 nm. An anode via 123 is etched through theSi₃N₄ directly over the cell body via, exposing the glass electrolyteelement 107.

In some arrangements, metal deposition and drive-in steps can beperformed after etching the anode via instead of before deposition ofthe second insulating layer 121 as described above.

In the embodiment of FIG. 2, the width of the anode via 123 ininsulating layer 121 is smaller than the width of the cell body 107 ininsulating layer 103, preferably by between about 10 nm and 100 nm andmore preferably by between about 10 nm and 60 nm. The anode via 123 ispositioned over the cell body 107 roughly concentrically, that is, sothat the sidewall of anode via 123 is spaced from the sidewall of cellbody 107 all the way around, and only a central portion of the cell body107 is exposed.

Referring now to FIG. 3, the anode via 123 and the cell body via 105have about the same size and are aligned directly over one another, inaccordance with another embodiment of the invention. Methods known inthe art can be used to avoid mask misalignment problems. Additionally, athin blanket layer 125 of spacer material, preferably an insulatingmaterial and most preferably Si₃N₄, is deposited conformally over theinsulating layer 121 and the anode via 123. The skilled artisan willappreciate, in view of the disclosure herein, that the spacer materialneed not be the same as the surrounding insulating layer, although it ispreferably a barrier to metal diffusion, particularly to diffusion ofthe fast diffusing element incorporated into the cell body 107 and anodeto be formed. The thickness of the spacer layer 125 is preferablybetween about 5 nm and 50 nm and more preferably between about 5 nm and30 nm.

Referring to FIG. 4, a spacer etch is performed, preferably by reactiveion etching (RIE), wherein horizontal portions 127 (FIG. 3) of thespacer layer 125 are removed preferentially, leaving vertical portionsof the spacer layer 125 relatively unaffected. FIG. 4 shows the verticalportions of the spacer layer 125 that remain after RIE, leaving a thespacer 131 lining vertical surfaces of the anode via 123. It will beunderstood that the spacer 131 forms a continuous lining around thesidewall of the anode via 123. In the illustrated embodiment, the spacer131 is a cylindrical annulus with a rounded top edge, whose outer sidesurface is in contact with the sidewall of the anode via 123.

Next, as shown in FIG. 5, a metal anode layer 133, preferably includinga metal or combination of metals from Group IB or Group IIB, morepreferably copper or zinc and most preferably silver, is deposited.Preferably, the metal anode layer 133 is deposited so that it fills theanode via 123 and forms a portion 135 overlying the second insulatinglayer 121 all as one contiguous body of material. The ovelying portion135 is subsequently patterned and etched as desired, depending upon thecircuit design of the memory array.

In FIG. 5, the metal deposition is shown for an anode via 123 with aspacer 131. The anode via filling and overlying anode layer can bedeposited in this same manner for the embodiment described with respectto FIG. 2, having an anode via 123 that is narrow (compared to the cellbody 107) without a spacer. In both the embodiment of FIG. 2 and theembodiment of FIG. 4, the anode makes contact with only a centralportion of the memory cell body and not with the sidewall edges.

When a voltage is applied across the lower electrode 101 and upperelectrode 133, a conductive path forms between the cathode 101 (i.e.,the electrode connected to the negative pole of the power supply) andthe anode 133 (i.e., the electrode connected to the positive pole of thepower supply). Without being limited by theory, it is believed that theconductive path grows by precipitation of cations (e.g., silver cations)from the memory cell body 107. Changes in the extent of the conductivepath affect the overall resistance of the device. The conductive pathtends to remain intact when the voltage is removed.

For a binary programmable conductor memory device, the memory has twobasic states: 0 and 1. When there is no conductive path, the memory cellhas high electrical resistance and reads as 0. When the conductive pathshorts the memory cell body 107, from the cathode 101 to the anode 133,the resistance is low and the memory cell reads as 1. The change inresistance of the memory cell with and without a conductive path can beas much as two orders of magnitude, e.g., a change from Megaohms tomilliohms. Reversing the polarity of the voltage reverses the formationof the conductive path, redissolving metal cations into the glass.

Alternatively, the memory cell can be programmed into as many as 3 or 4states by setting the extent of the conductive path growth. Thesechanges can be detected easily by the bit lines and word lines in amemory array, such that changing the extent of the conductive path canserve to change the state of the memory bit.

Thus, in one embodiment of the current invention, an anode via is madesmaller than the cell body via so that the overlying insulator layercovers the cell body/insulator interface. The smaller anode vias arepositioned so that their bottoms make contact only with the cell bodyand do not extend to the cell body/insulator interface. In anotherembodiment, a spacer prevents contact between the anode material and thecell body/insulator interface by covering the interface with spacermaterial near the outer edge of the anode via bottom. The preferredembodiments thus give reliable control to the spacing between the edgeof the anode and the edge of the memory cell body or GFID material.These structures ensure that the anode cations that precipitate out toform the conductive path are those that were intentionally andcontrollably provide to the glass electrolyte material, whether byphotodissolution, separate metal-contining layer (see FIG. 1 c),co-deposition or any other manner of metal doping. Silver contentdissolved within a GeSe glass, for example, is self limiting at about 30atm %, thus providing a reliably consistent source of diffusion ions forselectively forming the conductive path. For a given cation (e.g., Ag)concentration in solution, this provides conductive pathway formationreproducibly dependent upon voltage applied across the electrodes and/orswitching time.

Although the embodiments of the invention have been described in thecontext of a vertically built device, one of skill in the art willrecognize that this is not the only possible configuration or method forconstructing a programmable conductor memory cell.

1. A method of forming a programmable conductor memory cell comprising:forming a cathode; forming a glass electrolyte element in contact withthe cathode; forming an insulating layer over the glass electrolyteelement; forming an anode via in the insulating layer, thereby exposinga surface of the glass electrolyte element; depositing a layer of spacermaterial that conforms to contours of the anode via and the insulatinglayer; preferentially etching horizontal portions of the spacer materialto expose a central portion of the surface of the glass electrolyteelement; and depositing a layer of conducting material sufficientlythick to fill the anode via and to provide a conducting layer over theinsulting layer, thus forming an anode.
 2. The method of claim 1,wherein forming the glass electrolyte element comprises forming agermanium-selenium glass and introducing silver ions into the glass bydeposition of a silver layer over the glass and subsequently diffusingsilver from the silver layer into the glass.
 3. The method of claim 2,wherein diffusing silver into the glass comprises photodissolution. 4.The method of claim 1, wherein forming the glass electrolyte elementcomprises forming a first germanium selenide layer, an intervening metalselenide layer over the germanium selenide layer, and a second germaniumselenide layer over the intervening metal selenide layer.
 5. The methodof claim 1, wherein forming the insulating layer comprises depositingsilicon nitride.
 6. The method of claim 1, wherein the anode via isformed to a width between about 200 nm and 300 nm.
 7. The method ofclaim 1, wherein depositing the layer of spacer material comprisesdepositing a layer of insulating material.
 8. The method of claim 7,wherein depositing the layer of spacer materials comprises depositing alayer of silicon nitride.
 9. The method of claim 8, wherein the layer ofspacer material is deposited to a thickness between about 5 nm and 30nm.
 10. The method of claim 1, wherein preferentially etching comprisesreactive ion etching.
 11. The method of claim 1, wherein depositing alayer of conducting material comprises depositing silver.
 12. A methodof forming a programmable conductor memory cell comprising: forming acathode; forming a glass electrolyte element in contact with the cathodeand having a sidewall edge; forming an insulating layer over the glasselectrolyte element and covering the sidewall edge; forming an openingin the insulating layer, to expose a surface of the glass electrolyteelement without exposing the sidewall edge; and depositing a layer ofconducting material into the opening to contact only the central portionof the surface of the glass electrolyte element, thus forming an anode.13. The method of claim 12, wherein forming the opening comprises:etching a via through the insulating layer; blanket depositing a spacermaterial layer; and preferentially etching horizontal portions of thespacer material layer to expose the central portion of the surface ofthe glass electrolyte element.
 14. The method of claim 13, wherein thespacer material comprises an insulating material.
 15. The method ofclaim 12, wherein the insulating material is silicon nitride.
 16. Themethod of claim 12, wherein forming the opening in the insulating layercomprises patterning and etching using a mask with an opening smallerand in width than the glass electrolyte element and having the openingarranged concentrically over the glass electrolyte element.